In general, an automatic transmission of a motor vehicle includes a number of selectively engageable friction elements (referred to herein as clutches). Selective engagement/disengagement of the clutches establish speed ratios between the transmission input shaft and the transmission output shaft. In particular, shifting from a currently established speed ratio to a new speed ratio generally involves disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
The torque capacity of a clutch (on-coming or off-going) involved in a shift is controlled by the fluid pressure that a clutch trim valve supplies to the clutch. The clutch trim valve receives a main line fluid pressure and supplies the clutch with a clutch feed pressure developed from the main line fluid pressure. In a typical system, an electronic control module (ECM) provides a solenoid valve of the clutch trim valve with a control signal. The control signal controls a pilot pressure of the solenoid valve which in turn controls the clutch feed pressure supplied to the clutch.